Method for supplying a paint application device with paint

ABSTRACT

The invention relates to a method for supplying a paint application device ( 1 ) with paint, whereby a determined amount of paint is transported between two scrapers ( 10   a,    11   a,    10   b,    11   b ) through a scraper line ( 35   a,    35   b ), from a first scraper station ( 6   a,    6   b ) which can be connected to a paint supply source ( 2 ), to a second scraper station ( 7   a,    7   b ) which can be connected to the paint application device ( 1 ). Said scraper line ( 35   a,    35   b ) is cleaned on the return path from the second scraper station ( 7   a,    7   b ) to the first scraper station ( 6   a,    6   b ), by a cleaning product which is also transported between the two scrapers ( 10   a,    11   a,    10   b,    11   b ). In this way, the scraper line ( 35   a,    35   b ) can be cleaned in a reliable manner, with a simple embodiment of the scraper ( 10   a,    11   a,    10   b,    11   b ), thus rendering the inventive method especially useful when a colour change is carried out.

The invention relates to a method for supplying a paint applicationdevice with paint, in which

-   -   a) a given paint volume in each case is conveyed between two        pigs through a pig line from a first pig station connectable to        the paint supply source to a second pig station connectable to        the paint application device;    -   b) the pig line is cleaned on the return path of the pigs from        the second to the first pig station by means of a given quantity        of cleaning agent that is conveyed by at least one pig;    -   c) the pigs are conducted through the pig line by a pressurised        pushing medium.

A method of the above-mentioned type is known from EP 1 172 152 A1. Itis especially suitable for use wherever a frequent change of the type ofpaint processed by the paint application device, e.g. a change of thepaint colour, is to be expected. The quantity of paint required for apainting process is pushed through the pig line as a column of liquidwell delimited at its leading and trailing ends. In this way, firstly,the internal cylindrical surface of the pig line is contaminated onlyslightly or not at all by the paint being transported; secondly, therisk that contaminants will be introduced into the volume of painttransported between the pigs is comparatively low. However, it isnecessary, at least at given time intervals and especially when a colourchange is made, to clean the internal cylindrical surface of the pigline with a cleaning agent.

With the method known from EP 1 172 152 A1 a liquid pushing mediumretained in a closed line circuit is used, which pushing medium iscontaminated in the course of time and therefore must be exchanged atgiven time intervals. The cleaning agent used for cleaning the internalcylindrical surface of the pig line is transported inside one or bothpigs, which for this purpose are configured as “double pigs” having aninternal cavity. However, the relatively small quantity of cleaningagent that can be used with this method is not always reliablysufficient for completely cleaning the pig line. Moreover, theconstruction of the “double pigs” used in this case is relativelycomplicated.

Described in DE 198 30 029 A1 is a method for supplying a paintapplication device in which various paints to be applied one afteranother are introduced successively into the same paint supply line, theindividual paint liquid columns being separated from one another bypigs. In addition, a column consisting of cleaning liquid can be addedbetween two successive columns of paint liquid. With this known methodthe pigs do not move back and forth between two stations, but arereturned via a separate line. Again, with this known method the cleaningof the lines through which the paint moves is not always sufficient, inparticular when a colour change is made.

It is the object of the present invention so to configure a method ofthe above-mentioned type that good cleaning of the pig lines is possibleusing apparatus of the lowest possible cost and complexity.

This object is achieved according to the invention in that:

-   -   d) on its return from the second pig station to the first pig        station the cleaning agent is transported between the two pigs.

According to the invention, therefore, the “tandem” of two pigs is usedin a similar way on its return from the second to the first pig stationas on the outward path: a given volume of liquid is transported betweenthe two pigs. Whereas on the outward path this liquid is paint, on thereturn path of the pigs from the second to the first pig station thespace located between them is used for transporting cleaning liquid. Inthis case a comparatively large volume is now available for the cleaningliquid, so that a good cleaning effect can be reliably expected. Withthe present invention the pigs may be of very simple, conventionalconstruction and also do not need to be especially long.

Compressed air is expediently used as the pushing medium for the pigs.Compressed air introduces practically no contaminants into the systemand can be discharged into the atmosphere, so that the use of a closedpushing medium system, as in EP 1 172 152 A1, is not required.

A liquid solvent is expediently used as the cleaning agent.

If compressed air is used as the pushing medium, the velocity of thepigs can be simply adjusted by appropriately throttling the expulsion ofair from the flow paths preceding the pigs. The greater the throttlingeffect, the slower the movement of the pigs.

It is especially preferred, when introducing the paint into the spacebetween the two pigs in the first pig station, if the pressurised paintis used as the pushing medium for the leading pig.

In principle, the distance travelled by the leading pig could be used asa measure for the quantity of paint introduced into the space betweenthe two pigs, and the feeding of paint into this space could thereforebe ended when the leading pig has travelled a given distance. Moreaccurate, however, is the embodiment of the invention in which thequantity of paint used as the pushing medium is measured, and theintroduction of paint into the space between the two pigs is ended whenthe measured quantity of paint has been introduced, the trailing pig,together with the paint volume and the leading pig, then being moved bythe pushing medium. The measuring of the paint introduced in the spacebetween the pigs may be carried out with high precision outside the pigstation in the paint supply line.

It is also advantageous, when introducing the cleaning agent into thespace between the two pigs in the second pig station, if the pressurisedcleaning agent is used as the pushing medium.

Because the precise quantity of cleaning agent which is transported bythe two pigs on the return path to the first pig station is lesscritical, an embodiment of the invention is to be recommended in whichthe supply of cleaning agent to the space between the two pigs in thesecond pig station is ended when the leading pig has travelled a givendistance, the trailing pig, together with the volume of cleaning agentand the leading pig, then being moved by the pushing medium. In the caseof cleaning agents, therefore, the accuracy of the volumetricmeasurement effected by measuring the distance travelled by the leadingpig is entirely sufficient.

The pig stations, too, should be flushed with cleaning agent at leastwhen making a colour change.

In this case it is especially recommended that the pig stations beflushed alternately with cleaning agent and compressed air. Anespecially good cleaning effect is achieved by alternating, pulse-likecharging with liquid cleaning agent and compressed air.

In cases when the application of the paint to the tool to be painted isassisted by electrostatic forces, the paint application device includesan electrode that can be placed under high voltage. In the case of“internal charging”, in which the paint to be applied comes into contactwith the high-voltage electrode inside the paint application device, theproblem arises of galvanically separating the paint application deviceand the neighbouring system components from the paint supply source,which generally is at ground potential. In this connection it isrecommended that the high voltage be applied to the paint applicationdevice only when the pigs are located at a given minimum distanceoutside the pig stations in the pig line. The minimum distance is soselected that the required galvanic separation is ensured in thecorresponding section of pig line without the risk of an electricalshort-circuit.

In the case of paint application devices operating with high voltage itis also advantageous if the cleaning agent is fed to the componentswhich can be placed under high voltage via a line, and is conducted awayfrom these components via a line, the lengths of which lines areartificially increased through coiling in a particular area. The complexand costly pig method of galvanic separation is not, therefore, used forthese lines. Instead, a sufficiently high electrical resistance betweenthe system components under high voltage and the system components atground potential is ensured by an appropriate length of the lines, whichlength normally considerably exceeds the length of the lines requiredfor geometrical reasons.

An embodiment of the invention is elucidated in more detail below withreference to the drawings, in which:

FIG. 1 shows schematically a paint supply system having two parallelbranches in the paint supply to the spray nozzle;

FIG. 2 shows on a larger scale a pig station as used in the paint supplysystem of FIG. 1.

The paint supply system represented in the drawings, in particular inFIG. 1, is used to supply a spray nozzle 1 operating with internalcharging and shown at the top edge of FIG. 1 selectively with one of thepaints of different colours circulating in the paint supply lines 2shown at the bottom edge of FIG. 1. In the system shown there are sevensuch paint supply lines 2, so that seven paint colours can be processed.In addition, a solvent supply line 3, a discharge line 4 and acompressed air line 5 are disposed parallel to the paint supply lines 2.

The supply of paint from the paint supply lines 2 to the spray nozzle 1is effected via two parallel system branches. The suffix a is appendedto the reference symbols of the components belonging to the left-handsystem branch in FIG. 1, while the suffix b is appended to the referencesymbols of the components belonging to the right-hand system branch inFIG. 1. Because both branches are of identical construction, only thesystem branch located on the left in FIG. 1 will be described in detailbelow.

This system branch comprises as its most important components a firstpig station 6 a located in the vicinity of the paint supply lines 2, anda second pig station 7 a located in the vicinity of the spray nozzle 1.The construction of all the pig stations 6 a, 6 b, 7 a, 7 b in the paintsupply system is identical, so that it is sufficient to elucidate indetail the construction of pig station 6 a with reference to FIG. 2:

The pig station 6 a comprises a housing 8 a in which is formed amovement passage 9 a for two pigs 10 a, 11 a arranged one behind theother. In FIGS. 1 and 2 the two pigs 10 a and 11 a are shown in theirrespective parking positions inside the pig station 6 a. Located inproximity to these parking positions are detectors 12 a, 13 a which canrespectively detect the presence of pig 10 a and pig 11 a in theirparking positions.

A total of four passages 14 a, 15 a, 16 a, 17 a, via which differentmedia can be introduced at different sites in the movement passage 9 ain a manner still to be described, lead through the housing 8 a to themovement passage 9 a. The middle passage 15 a in FIG. 2 leads to the endof the movement passage 9 a, so that the medium conducted to this sitecan act upon the end face of the pig 11 a located at the bottom in FIG.2. The other passages 14 a, 16 a, 17 a, by contrast, open from oppositesides into the movement passage 9 a at a site located between the twopigs 10 a and 11 a, so that the space located between these two pigs 10a and 11 a can be reached from here. Located in each of these threepassages 14 a, 16 a, 17 a is a respective stop valve 67 a, 18 a, 19 a.

A stop 20 a actuated by compressed air can be moved into the movementpassage 9 a of the pigs 10 a, 11 a. Only when the stop 20 a is retractedcan the pigs 10 a, 11 a be moved out of or into the pig station 6 a.

As shown in FIG. 1, the bottom, left-hand passage 14 a of the pigstation 6 a in FIG. 2 is connected to the solvent supply line 3 via aline 21 a in which a stop valve 22 a is located. The passage 17 locatedat the top left in FIG. 2 is connected to the compressed air line 5 viaa line 23 a in which a stop valve 24 a is located. The passage 16 alocated at the bottom right in FIG. 2 is connected to a colour-changeunit 27 a via a line 25 a in which a volumeter unit 26 a is located. Thecolour-change unit 27 a is in turn in communication, via a total of ninebranch lines 28 a, with the paint supply lines 2, with the solventsupply line 3 and with the discharge line 4. The colour-change unit 27 ais able to establish a connection selectively between the line 25 a andone of the lines 2, 3, 4.

Finally, the passage 15 a located in the lower middle portion of thehousing 8 a of the pig station 6 a is connected to a change-over valve31 a via a line 29 a in which a controllable throttle valve 30 a islocated. The change-over valve 31 a is able to connect the line 29 aselectively to a first branch line 32 a or a second branch line 33 a, orto interrupt both connections. The left-hand branch line 31 a in FIG. 1leads via a stop valve 33 a to the compressed air supply line 5, whilethe right-hand branch line 32 a in FIG. 1 leads via a stop valve 34 a tothe discharge line 4.

The mouth of the movement passage 9 a of the pig station 6 a isconnected to the mouth of the movement passage 9 a of the oppositelyarranged pig station 7 a located in proximity to the spray nozzle 1 viaa pig line 35 a represented only schematically in FIG. 1. The pig line35 a may be a flexible hose the internal diameter of which is adapted inknown fashion to the external diameter of the pigs 10 a, 11 a in such away that the cylindrical surfaces of the pigs 10 a, 11 a fit in afluid-tight manner against the internal cylindrical surface of the pigline 35 a as they move through same.

The different passages 9 a, 14 a, 15 a, 16 a and 17 a of the pig station7 a close to the spray nozzle are integrated in the system as follows:

The passage 17 a is connected via a line 36 a to a compressed airdistribution line 37 which in turn is connected via a stop valve 38 tothe compressed air line 5.

The passage 14 a of the pig station 7 a is connected via a line 39 a toa solvent distribution line 40 a which is connected via a stop valve 41to the solvent supply line 3. The solvent distribution line 40 is woundto form a spiral 42 at one location. For reasons which will become clearbelow the overall length of the solvent distribution line 40 is therebyintended to be increased.

The passage 15 a of the pig station 7 a close to the spray nozzle is inturn connected to a change-over valve 45 a via a line 43 a in which acontrollable throttle valve 44 a is located. The change-over valve 45 ais able to connect the line 43 a selectively to one of two lines 46 a,47 a, or to shut off the line 43 a. The upper line 46 a in FIG. 1 leadsto a disposal collection line 48 which in turn is connected to thedisposal line 4 via a spiral-wound portion 49 and a stop valve 50.

Finally, the passage 16 a of the pig station 7 a close to the spraynozzle is connected via a line 50 a to a further change-over valve 51 towhich the line 50 b of the right-hand system branch in FIG. 1,corresponding to the line 50 a, also leads. The two system branches arethereby brought together at the change-over valve 51. The change-overvalve 51 is able to connect each of the lines of 50 a, 50 b selectivelyto one of four lines 52, 53, 54, 55 or to shut off each of said lines 50a, 50 b. The bottom line 52 in FIG. 1 leads to the disposal collectionline 48, the line 53 located above it to the solvent distribution line40 and the line 54 located above it again to the compressed airdistribution line 37, while the line 55 extending substantially upwardlyfrom the change-over valve 51 leads to a metering pump 56, the outlet ofwhich is in turn connected to the spray nozzle 1. The metering pump 56may in addition be supplied with solvent from the solvent distributionline 40 via a line 57. Finally, the spray nozzle 1 is connected to thedisposal collection line 48 via a further line 58.

In the following description of the operation of the paint supplysystem, the right-hand system branch in FIG. 1 containing the componentsdesignated by b will initially not be considered. The way in which thissystem branch contributes to the overall operation will then beexplained.

The situation represented in FIG. 1, in which the pigs 10 a, 11 a arelocated in the pig station 6 a situated in the vicinity of the paintsupply lines 2 will be taken as the starting point. Their presence inthat location is verified by the detectors 12 a, 13 a. The stop 20 a hasbeen moved into the movement path of the pigs 10 a, 11 a, so that theycannot leave this pig station 6 a. It should also be assumed that paintresidues originating from an earlier painting process have been cleanedfrom all components in a manner which is not yet of interest here. For anew painting process a given quantity of the paint supplied to one ofthe paint supply lines 2 must now be delivered therefrom to the spraynozzle 1. To achieve this, the following procedure takes place:

First, by opening the corresponding stop valve in the colour-change unit27 a, a connection is established between the desired paint supply line2 and the line 25 a leading to the passage 16 a of the pig station 6 a.The stop 20 a is retracted so that nothing now prevents the upper pig 10a from moving out of the pig station 6 a. By opening the valve 18 a inthe pig station 6 a, paint is now enabled to enter the space between thetwo pigs 10 a and 11 a, pushing the upper pig 10 a in FIG. 1 out of thepig station 6 a.

As this happens, the pig 10 a displaces the air located ahead of it inthe direction of movement in the pig line 35 a. This air is fed via themovement passage 9 a of the pig station 7 a close to the spray nozzle,via the latter's passage 15 a and the line 43 a and, with appropriatepositioning of the change-over valve 45 a, via the line 46 a and thedischarge line 48, with the stop valve 50 open, to the discharge line 4.As this happens, the throttle valve 44 a located downstream of the pigstation 7 a which is close to the spray nozzle, is so adjusted that thedesired movement velocity of the pig 10 a in the pig line 35 a isproduced.

The quantity of paint which is dispensed into the space between themoving pig 10 a and the pig 11 a, still located in its parking stationin the pig station 6 a, is monitored by the volumeter device 26 a. Oncethe desired quantity is reached, both the corresponding stop valve inthe colour-change unit 27 a and the stop valve 18 a in the pig station 6a are closed. The second pig 11 a is now connected by its lower end facein the drawings, via the line 29 a and the correspondingly setchange-over valve 31 a, to the compressed air line 5, after the stopvalve 33 a has been opened. The compressed air now also pushes the pig11 a out of the pig station 6 a and—via the paint enclosed between thetwo pigs 10 a and 11 a—pushes forwards the pig 10 a which has first leftthe pig station 6 a and which up to this time has been propelledforwards by the paint.

A kind of “package” comprising the two pigs 10 a and 11 a and the paintvolume enclosed therebetween is now formed and is moved forwards in thepig line 35 a by the compressed air supplied via the line 29 a. As thishappens the throttle valve 30 a in the line 29 a is completely opened.

Located at a given distance from the outlet of the pig station 6 a is afurther detector 59 which can detect the passing-by of the two pigs 10a, 11 a. The distance between the detector 59 and the pig station 6 a issuch that sufficient electrical insulation is achieved by the length ofthe corresponding section of the pig line 35 a. The high voltage can nowbe applied to the internal electrode of the spray nozzle 1.

After passing through the pig line 35 a, the leading pig 10 a firstenters the pig station 7 a located close to the spray nozzle; as thishappens, the stop 20 a of the pig station 7 a must, of course, beretracted. The reaching of its end and parking position by the pig 10 ais detected by the detector 13 a of the pig station 7 a. The connectionto the discharge line 48 is now interrupted in the change-over valve 45a. At the same time, by suitable switching of the change-over valve 51,the line 50 a is connected to the metering pump 56 via the line 55. Asthe second pig 11 a, pushing the paint volume before it, approaches thepig 10 a, which has come to a stop in its parking position in the pigstation 7 a, the paint volume is displaced via the lines 50 a and 55 tothe metering pump 56. By appropriate actuation of the spray nozzle 1,the workpiece, for example, a vehicle body, can now be painted. Thequantity of paint required in each case is set by the metering pump 56.

Once the painting process is completed, the high voltage is disconnectedfrom the spray nozzle 1. The spray nozzle 1, the metering pump 56 andthe line 55 between metering pump 56 and change-over valve 51 areflushed via the lines 53 and 57, with suitable setting of thechange-over valve 51, and via the line 58, with the stop valves 41 and50 open.

The residual paint still remaining between the pigs 11 a and 10 a in thepig station 7 a is discharged by setting the change-over valve 51 sothat the line 50 a is now connected to the line 52 and therefore to thedisposal line 4.

When the detector 12 a of the pig station 7 a located close to the spraynozzle detects that the second pig 11 a has also reached its parkingposition inside the pig station 7 a, the stop 20 a of the pig station 7a is moved out, retaining the two pigs 10 a, 11 a in the pig station 7 alocated close to the spray nozzle.

The paint in the line 50 a, which connects the pig station 7 a to thechange-over valve 51, is then discharged as follows: the valves 67 a and18 a of the pig station 7 a are opened and the change-over valve 51 isactuated in such a way that a connection is established between the line50 a and the discharge collection line 48. In this way solvent can flowthrough the space located between the two pigs 10 a, 11 a and the line50 a and clean the corresponding paths. By alternately opening thevalves 19 a and 67 a the flow can be effected in a pulsed manneralternately with compressed air and with solvent. To conclude thiscleaning process, any solvent located between the pig station 7 a andthe change-over valve 51 is expelled with compressed air.

The transporting of the two pigs 10 a, 11 a from the pig station 7 aclose to the spray nozzle back to the pig station 6 a close to the paintsupply lines 2 can now begin. As this happens, cleaning of theconnecting path between the two pig stations 7 a, 6 a, in particular ofthe pig line 35 a, takes place. Once again, a “package” is formed by thetwo pigs 10 a and 11 a and a volume of liquid enclosed thereby. However,this liquid is now a cleaning solvent. The individual processes are asfollows:

The stop 20 a of the pig station 7 a is first retracted, clearing theway for the pigs 10 a, 11 a. The throttle valve 30 a located downstreamof the pig station 6 a is now so adjusted that a certain resistance isproduced for the air to be displaced, which is located in the pig line35 a, determining the movement velocity of the pigs 10 a, 11 a and ofthe volume of solvent enclosed therebetween.

First, by opening the valve 67 a of the pig station 7 a with the stopvalve 41 open, solvent is introduced into the space between the two pigs10 a and 11 a via the solvent distribution line 40 and the line 39 a.The pig 11 a, which is leading in this case, is thereby forced out ofthe pig station 7 a. A further detector 60 a, which responds to thepassing-by of the two pigs 10 a, 11 a, is fitted at a given distancefrom the pig station 7 a in proximity to the pig line 35 a. If thedetector 60 a detects that the leading pig 11 a has passed thecorresponding point in the pig line 35 a, the valve 67 a is closed andthe further supply of solvent to the space between the two pigs 10 a, 10b is interrupted.

Compressed air is now supplied to the upper end face in FIG. 1 of thepig 10 a still located in the pig station 7 a via the change-over valve45 a, with the throttle valve 44 a substantially open, and via thecompressed air distribution line 37 and the lines 47 a and 43 a. Thiscompressed air now pushes the entire “package” consisting of the twopigs 10 a, 10 b and the enclosed volume of solvent through the pig line35 a. Once the trailing pig 10 a has passed the detector 60 a asufficient insulating distance between the “package” and the pig station7 a is present, so that the high voltage can again be applied to thespray nozzle 1.

Finally, the leading pig 11 a in this cleaning process enters the pigstation 6 a close to the supply lines 2. If the detector 13 a of the pigstation 6 a detects that the pig 11 a has again reached its parkingposition represented in FIG. 1, the connection between the line 29 a andthe discharge line 4 is interrupted in the change-over valve 31 a.Instead, the valve 18 a of the pig station 6 a and the correspondingvalve inside the colour-change unit 27 a are opened in such a way thatthe volume of solvent enclosed between the two pigs 10 a, 11 a can beforced into the discharge line 4 via the line 25 a and the colour-changeunit 27 a. At the same time, the connecting line 25 a and the volumeterunit 26 a located therein are cleansed of paint.

If the detector 12 a of the pig station 6 a detects that the trailingpig 10 a has also moved into its parking position in the pig station 6a, the stop 20 a of the pig station 6 a is moved in, so that both pigs10 a, 11 a are retained in the pig station 6 a. By opening the stopvalve 22 a in the line 21 a and the valve 67 a in the pig station 6 a,the flushing process can be continued. As this happens, cleaning mayagain be carried out in a pulsed manner alternately with compressed airand with solvent by alternately opening the valves 67 a and 19 a of thepig station 6 a. The final flushing process should again be carried outwith compressed air.

The valves 18 a of the pig station 6 a and the stop valve of thecolour-change unit 27 a leading to the discharge line 4 are now closed.The left-hand system branch in FIG. 1 has now been completely cleanedand is ready for a new painting process using the same or anothercolour.

In principle, the paint supply system can be operated in theabove-described manner with a single system branch. However, because ofthe return transportation of the two pigs 10 a, 11 a from the pigstation 7 a close to the spray nozzle to the pig station 6 a close tothe paint supply lines 2 and the associated cleaning process, undesiredpauses in the painting process occur. For this reason, the second systembranch which, as already mentioned, is constructed identically to thefirst system branch, is provided in the embodiment represented in thedrawings. The two system branches are operated in a counter-cyclingmanner in the sense that one is always in the mode in which paint istransported in the direction of the spray nozzle 1 while the other is inthe cleaning mode, in which the corresponding pig line 35 a or 35 b andthe other components of the respective system branch are being freed ofpaint residues.

The spirals 42 and 49 in the distribution/collection lines 40 and 48 areintended to have the following effect: via the distribution/collectionlines 40 and 48 there is a direct connection between the high-voltageelectrode of the spray nozzle 1 and the solvent supply line 3 and thedischarge line 4, which are connected to ground potential. To avoid anelectrical short-circuit here the lengths of the distribution/collectionlines 40, 48 are increased by the spirals 42 and 49 to such an extentthat the electrical resistance formed thereby prevents the electricalshort-circuit.

If no colour change is to take place between painting processes, theabove-described processes can be carried out analogously, althoughcleaning processes are omitted.

1. Method for supplying a paint application device with paint, in whicha given paint volume in each case is conveyed between two pigs through apig line from a first pig station connectable to the paint supply sourceto a second pig station connectable to the paint application device, thepig line is cleaned on the return path of the pigs from the second tothe first pig station by means of a given quantity of cleaning agentthat is conveyed by at least one pig, the pigs are conducted through thepig line by a pressurised pushing medium, the method comprising: onreturn from the second pig station to the first pig station, thecleaning agent is transported between the two pigs.
 2. Method accordingto claim 1, wherien a liquid solvent is used as the cleaning agent. 3.Method according to claim 1, wherein compressed air is used as thepushing medium for the pigs.
 4. Method according to claim 3, whereinvelocity of the pigs is adjusted by appropriately throttling theexpulsion of air from the flow paths located ahead of the pig.
 5. Methodaccording to claim 1, wherein as the paint is introduced into the spacebetween the two pigs in the first pig station the pressurised paint isused as the pushing medium for the leading pig.
 6. Method according toclaim 5, wherein quantity of paint used as the pushing medium ismeasured and the supply of paint to the space between the two pigs isended when the desired quantity of paint has been introduced, and inthat the trailing pig is ended when the desired quantity of paint hasbeen introduced, and in that the trailing pig, together with the paintvolume and the leading pig, is then moved by the pushing medium. 7.Method according to claim 1, wherein as the cleaning agent is introducedinto the space between the two pigs in the second pig station, thepressurised cleaning agent is used as the pushing medium.
 8. Methodaccording to claim 7, wherein supply of cleaning agent to the spacebetween the two pigs in the second pig station is ended when the leadingpig has moved a given distance, and in that the trailing pig, togetherwith the cleaning agent and the leading pig, is moved by the pushingmedium.
 9. Method according to claim 1, wherein the pig stations areflushed with cleaning agent at least when a colour change is made. 10.Method according to claim 9, wherein the pig stations are flushedalternately with cleaning agent and compressed air.
 11. Method accordingto any claim 1, wherein the paint application device includes anelectrode that is connectable to a high voltage, and further wherein thehigh voltage is applied to the paint application device only when thepigs are located at a given minimum distance outside the pig stations inthe pig line.
 12. Method according to claim 11, wherein the cleaningagent is fed to the components that are connectable to a high voltagevia a line and is conducted away from these components via a line, thelengths of which lines are artificially increased by coiling in aparticular area.